Process for runnerless injection - compression molding of thermosetting materials

ABSTRACT

A method and apparatus are described. A thermosetting molding material, suitably a synthetic resin composition such as a phenolic molding composition, is initially heated and plasticized. The plasticized mass is passed as a stream through an injection nozzle which is maintained at a temperature sufficiently high to keep the mass in a plasticized condition but sufficiently low to avoid any substantial curing or setting-up of the mass. The plasticized mass is next passed through a temperature-controlled zone maintained at a temperature sufficiently high to keep the mass in a plasticized condition but sufficiently low to avoid any substantial curing or setting-up of the mass. The plasticized stream is passed into a partially open, heated mold. The mold cavity is maintained at temperatures sufficiently high to cure the thermosetting molding material. When the partially open mold is partially full, the feed steam is interrupted, and the mold is then completely closed to press and cure the enclosed plasticized material. The cured, finished article is removed from the mold and the process repeated. 
     An apparatus for conducting the process of the invention includes a stationary assembly and a movable assembly. The stationary assembly is comprised of a stationary platen which has a heat plate, a temperature-controlled sprue bushing and a mold face positioned thereon. The movable assembly is comprised of a movable platen which has a heat place mounted thereon and a mold face, corresponding to, and adapted to be aligned with, the mold face on the stationary assembly to form a partially open mold cavity. Means are provided to feed a supply of thermosetting molding material through the sprue bushing into the partially open mold cavity. Means are provided to completely close the mold cavity and place the apparatus in a compression stage. Means are also provided for the subsequent retraction of the movable assembly to allow removal of a molded article and access to the mold area.

The present application is a division of U.S. Ser. No. 278,238, filedJune 29, 1981, now U.S. Pat. No. 4,370,123, which is acontinuation-in-part of U.S. Ser. No. 118,281, filed Feb. 5, 1980, nowU.S. Pat. No. 4,290,744, in turn a continuation-in-part of U.S. Ser. No.972,182, filed Dec. 21, 1978, now abandoned, all entitled, "Apparatusfor Runnerless Injection--Compression Molding Thermosetting Materials",and said application Ser. No. 278,238 also being a continuation-in-partof U.S. Ser. No. 164,412, filed June 30, 1980, now U.S. Pat. No.4,309,379 in turn a continuation-in-part of U.S. Ser. No. 972,189, filedDec. 21, 1978, now U.S. Pat. No. 4,238,181, both entitled, "Method andApparatus for Runnerless--Compression Molding Thermosetting Materials".

BACKGROUND OF THE INVENTION

The present invention relates to a process for molding thermosettingmaterials and, more particularly, to the molding of thermosettingsynthetic resin compositions.

Injection molding is an advantageous method of forming articles ofsynthetic resin. In general terms, injection molding is a processwherein the molding composition, in a melted or plasticized state, isinjected into a mold cavity. Typically, molding composition in granularform is fed from a hopper into a heated cylinder containing a screw, orram. The molding composition is heated, melted and plasticized in theheated cylinder area, and then the screw, or ram, injects the melted andplasticized material into a mold cavity. In the case of thermosettingmaterial, the molded material is cured in the mold by compaction and byheat added to the mold cavity. After curing, the article is ejected fromthe mold and the process repeated.

Injection molding offers the advantage of reduced molding cycles, bettercontrol of process variables and increased productivity as compared withconventional compression and transfer molding processes. The majordisadvantage with the injection molding of thermosetting materials isthe generation of a considerable amount of waste material. The wastematerial is generated by thermosetting material that has cured, i.e.become infusible, in the sprue system and cannot be reused. The amountof non-reusable waste material generated in this fashion in singlecavity molding can be substantial, ranging typically from about 2 toabout 15 percent of the total amount of material required to mold anarticle.

A more recent technical advance in the molding art has been theadaptation of the runnerless injection, or cold manifold, process to theinjection molding of thermosetting resins. In the cold manifold process,the material in the sprue and manifold system is maintained at atemperatuare sufficient to maintain the material in a plasticizedcondition, without causing the material to prematurely cure, or"set-up". Thus, when a cured part is removed from the mold cavity, thematerial in the sprue and manifold becomes part of the next molding,instead of being discarded as in conventional injection moldingoperations. The runnerless injection proces, therefore, provides forsignificant savings in material.

The thermosetting materials usually employed in runnerless injectionprocesses differ in some respects from materials normally employed inconventional injection processes because of the different requirementsof each process. One significant difference is that a standard injectionmolding material typically has a softer plasticity. In contrast, arunnerless injection material is adapted to remain in a plasticized, orfused, condition in the feed system for extended periods of time withoutprematurely curing, usually at temperatures between about 104° and 116°C. (220° to 240° F.), while also being capable of rapidly curing in themold cavity at the molding temperature, usually about 170° C. (340° F.).Examples of suitable runnerless injection molding compositions aredescribed in U.S. Pat. Nos. 4,210,732; 4,239,869; and 4,241,201, allentitled, "Phenolic Resins With Improved Low Temperature ProcessingStability". The disclosures in the forgoing patents are herebyincorporated by reference. Although such formulations are useful in thepresent apparatus, they are not required, and the molding compositionspresently utilized may be selected from the more economical and morereadily available standard thermosetting molding compositions.

Thermosetting molding materials useful in the present invention maysuitably be selected from thermosetting synthetic resins and resincompositions typically used in molding operations; for example,phenolic; amino, such as urea, melamine and melamine/phenolic; polyesterresins in granular, nodular, bulk or sheet forms; alkyd; epoxy;silicone; diallylphthalate; polyamides; or from thermosetting naturaland synthetic rubber compositions. Phenolic resin compositions areespecially useful as the feed material. Phenolic resin compositions usedin molding operations are usually employed in the form of moldingcompositions typically are particulate in form, containing a moldinggrade phenolic resin, a cross-linking agent, such ashexamethylenetetramine, and suitable filler materials.

The technique of injection--compression molding basically consists ofinjecting a charge of plasticized molding material into a partially openmold; the final fill, or mold fit, is accomplished by the subsequentcomplete closure of the mold. Injection-compression molding makespossible a combination of the positive attributes of compressionmolding, i.e., improved dimensional stability, uniform density, lowshrinkage and high impact strength with the advantages of automation andfast cure of injection molding.

The present invention provides an improved method and apparatus foradapting the advantages of injection-compression molding to include theadvantages of runnerless injection techniques and facilitates the use ofstandard phenolic molding compositions in such apparatus. The presentapparatus allows the use of commercially available, standard nozzles inrunnerless injection-compression molding processes. Heretofore,adaptations were required in the nozzle and distribution system toobtain the foregoing advantages.

GENERAL DESCRIPTION OF THE INVENTION

In accord with the present invention, a thermosetting molding material,suitably a synthetic resin composition such as a phenolic moldingcomposition, is initially heated and plasticized. The plasticized massis then passed as a stream through a nozzle which is maintained at atemperature sufficiently high to keep the mass in a plasticizedcondition but sufficiently low to avoid any substantial curing orsetting-up of the mass. The plasticized stream is subsequently injectedinto a partially open, heated mold. The mold cavity is maintained attemperatures sufficiently high to cure the thermosetting moldingmaterial. When the partially open mold is partially full, the feedstream in interrupted, and the mold is then completely closed to pressand cure the plasticized material enclosed therein. The cured, finishedarticle is removed from the mold and the process repeated.

The direct, positive mold system of the present invention is comprisedof two interfitting halves, or mold faces, which, when joined inregister, define the mold cavity. The mold cavity, when fully closed,has the volume and configuration substantially identical to the desiredfinished molded article. In accord with the injection-compressiontechnique employed in the present invention, the mold faces are notfully closed when the plasticized material is injected into the moldcavity. Thus, filling of the mold cavity is carried out against littleor no backpressure. The mold faces are heated to a temperaturesufficiently high to cure the thermosetting molding material. The massthat is enclosed within the mold should remain sufficiently plastic sothat, upon complete closure of the mold, the material will flow to fillevery part of the mold cavity.

The present invention combines the positive attributes of runnerlessinjection and compression molding techniques which provides increasedproductivity and the production of products having improvedcapabilities. The present invention also provides an improved means tomaintain the plasticized mass within the system in an uncured state forsubstantially longer periods of time than the distribution systems ofthe prior art. This is of particular importance when extra time isrequired to clean or clear a mold, or when the operation is interruptedbecause of equipment failure.

The present invention utilizes an improved molding apparatus forinjection-compression molding of thermosetting molding materials. Theapparatus comprises a stationary platen assembly and a movable platenassembly.

The stationary assembly is comprised of a stationary platen member, orsupport plate, a heated plate mounted on the support plate and a moldface mounted on the heat plate. The stationary assembly has a cooledsprue bushing positioned therein and extending therethrough. The spruemember has a receiving end adapted to engage, usually by contact, withthe nozzle member of an injection molding machine. The sprue bushing hasa discharge end adapted to feed thermosetting molding material directlyinto the mold cavity. The temperature maintained in the sprue bushing issufficiently high to maintain the thermosetting molding material withinthe sprue bushing in a plasticized state and sufficiently low that nosubstantial curing, or setting-up, of the thermosetting molding materialtakes place. By substantial curing or setting-up of the thermosettingmolding material is meant the amount of premature polymerization whichwould adversely inhibit the plasticity or mobility of the moldingmaterial.

The movable assembly is comprised of a movable platen member, or supportplate, a heated plate mounted thereon and a mold face mounted on theheat plate, corresponding to and adapted to be aligned with, the moldface on the stationary assembly to form a mold cavity.

In the injection, or feed, stage the movable assembly is positioned tobring the mold face on the movable assembly into close proximity withthe corresponding and aligned mold face on the stationary assembly toform a partially open mold cavity. Thermosetting molding material in aplasticized state is then fed through the nozzle member, through thecooled sprue bushing, directly into the partially open, heated moldcavity. In the injection stage and in the subsequent compression stage,the discharge end of the sprue bushing is in direct feed relationshipwith the mold cavity, allowing flow of molding material from the spruebushing into the mold cavity in the injection stage and flow of moldingmaterial from the mold cavity into the sprue bushing in the compressionstage.

In the compression stage, the heated mold cavity is closed completely byfurther movement, suitably by hydraulic and/or mechanical means, of themovable assembly in a direction toward the stationary assembly. Thismovement creates a back-pressure at, and a back-flow through, thedischarge end of the sprue member. Thus, much of the molding materialwhich would normally be lost in the sprue, and flash as waste issalvaged by back-flow of the material into the cooled sprue bushing,making the material available for the next shot, or filling, uponclosing of the mold faces, the thermosetting material within theconfines of the mold cavity is pressed and heated to a temperaturesufficiently high to cure the material.

After curing, the movable assembly is retracted, moved away from, thestationary assembly, suitably by hydraulic and/or mechanical means, toremove the molded article and provide access to the mold face andsurrounding area. The thermosetting material remaining in the nozzle andsprue is maintained in a plasticized, substantially uncured state, readyfor the next shot or filling.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be illustrated and more fully described byreference to the accompanying drawings.

FIG. 1 is a cross-sectional, partly schematic view of an apparatus ofthe present invention. The apparatus shown in a portion of an injectionmolding machine having a horizontal clamping arrangement. As shown inFIG. 1, the apparatus is in the injection, or feed, stage of the moldingcycle. In this stage, thermosetting molding material is fed into apartially open mold.

FIG. 2 shows the apparatus of FIG. 1 in the compression stage. In thisstage, the mold is completely closed. The thermosetting material withinthe confines of the closed mold during this stage is pressed and heatedto cure the enclosed material to the shape of the mold cavity.

FIG. 3 shows the apparatus of FIG. 1 in an open position. In thisposition, the movable platen assembly has been actuated to move in adirection away from the stationary platen assembly, allowing the removalof the molded article from the open mold face and surrounding area inpreparation for repositioning the components in the feed stage as shownin FIG. 1.

Looking now at FIG. 1, thermosetting molding material is fed into feedhopper 11 and then into a plasticizing zone formed by heated barrel 13and by the mechanical working of screw 15. A predetermined amount ofplasticized molding material is subsequently injected by screw 15,acting as a ram, through nozzle member 17, through cooled sprue bushing19, into a single mold cavity 23, formed by partially open mold faces 21and 25.

The stationary assembly is comprised of stationary platen, or supportplate, 27, which has stationary heat plate 29 mounted thereon. Suitably,heat place 29 is separated from platen 27 by a layer of insulation 31,e.g., insulation board. Heat plate 29 may suitably be heated by thecirculation of steam or hot oil therethrough but, more preferably,electrically. Heat plate 29 supplied heat to mold face 21. Thetemperataure maintained on the mold face is sufficiently high that, whencombined with the pressure generated when the mold is completely close,will cure, or set, the thermosetting material within the mold cavity.Temperatures in a range between about 135° and about 216° C. (275° to425° F.) are generally useful, and, within that range, temperaturesbetween about 149° and about 199° C. (300° to 390° F.) are particularlyuseful for a wide variety of thermosetting molding materials.

The stationary assembly has sprue bushing 19 therein and extendingtherethrough. Sprue bushing 19 has a receiving end 33 adapted to engage,by contact, nozzle member 17. Sprue bushing 19 has a discharge end 35opening directly into, in direct feed relationship to, mold cavity 23formed by mold faces 21 and 25. Sprue bushing 19 has a cooling meanstherein to maintain a temperature within the sprue member below thetemperature at which any substantial curing of the thermosetting moldingmaterial will occur and sufficiently high to maintain the thermosettingmaterial in a plasticized condition. Temperatures below which anysubstantial curing takes place are temperatures which are sufficientlylow that the fluidity of the plasticized thermosetting feed material isnot permanently affected. Generally, temperatures in the range betweenabout 77° and about 143° C. (170° to 290° F.) are useful, andtemperatures from about 99° to about 116° C. (210° to 240° F.) arepreferably maintained.

Suitably, sprue bushing 19 is cooled by the circulation of a liquid,such as water, at the desired temperature through internal cavities, orchannels, such as 37.

The movable assembly consists of movable platen, or support plate, 39,which has heat plate 41 mounted thereon and movable therewith. Heatplate 41 has a mold face 25 mounted thereon, positioned to align inregister with corresponding mold face 21 of the stationary platenassembly. Support blocks, such as 43, 45 and 47, are suitably utilizedbetween platen 39 and heat plate 41 to provide a means of removing themolded article from the mold, for example, knock-out rods or pins, notshown. The movable assembly is adapted to be moved reciprocally as aunit, by means not shown, but suitably hydraulically and/ormechanically, in and out of aligned contact with the stationary platenassembly.

Movable heat plate 41 is suitably heated by the circulation of steam orheated oil therethrough but, more practically, electrically, to maintaina temperature on mold face 25 comparable to that maintained on thecorresponding mold face 21, that is, a temperature sufficiently high tocure the thermosetting material upon complete closure of the mold faces.

In the injection stage as shown in FIG. 1, mold faces 21 and 25 formpartly open mold cavity 23. The plasticized molding material enters moldcavity 23. The plasticized molding material enters mold cavity 23directly from discharge end 33 of sprue bushing 19 and fills partiallyopen mold cavity 23. The distance the mold faces move from a partiallyopen position to a fully closed position generally ranges between about0.06 and about 0.50 inches and, more preferably, between about 0.10 andabout 0.2 inches.

FIG. 2 shows the apparatus of FIG. 1 in a closed or compression stage ofthe molding cycle. In this stage, movable platen assembly has movedtoward the stationary platen assembly to close aligned mold faces 21 and25. Upon closure, excess molding material in the mold cavity 23 isforced, or fed, directly back through discharge end 35 of cooled spruebushing 19 to be used in the next shot, or fill. Only a minor amount,less than 1.0 percent, of the molding material is forced out the sidesof the mold faces to be lost as flash. In the closed, or compression,stage the thermosetting molding material previously fed into the spacebetween the partially open mold faces is pressed and heated to cure theenclosed material to the internal shape of the mold.

FIG. 3 shows the apparatus of FIG. 1 in an open position. In thisposition, the movable platen assembly has been retracted, or moved awayfrom, the stationary assembly. The molded article 49 is removed from themold, suitably by the aid of knock-out pins, not shown, which areusually positioned in the movable mold faces. In this position, anyflash which may be present is removed and, if necessary, the mold facecleaned. The components are then ready for repositioning as shown inFIG. 1. At the point shown in FIG. 3, the thermosetting molding materialin the nozzle and sprue bushing is stored in a plasticized,substantially uncured state in preparation for the next shot, orinjection, of thermosetting material into the mold cavity.

For purposes of simplicity, the present invention has been described interms of a horizontal clamping arrangement; however, it will beappreciated and understood that the invention is equally adapted to, anduseful in, vertical clamping arrangements.

The foregoing description and embodiments are intended to illustrate theinvention without limiting it thereby. It will be appreciated thatvarious modifications can be made in the invention without departingfrom the spirit and/or scope thereof.

In the above-described drawing, the visible edges and exposed surfacesbehind the cutting plane have been omitted in the vicinity of the moldedcavities to simplify the drawings and facilitate the understanding ofthe apparatus of the invention.

We claim:
 1. A method of injection-compression molding an article fromthermosetting material comprising the steps of:(a) plasticizing apredetermined amount of thermosetting molding material, (b) passing saidamount of plasticized molding material through a temperature-controlledzone maintained at a temperature sufficiently low to prevent substantialcuring of the molding material and sufficiently high to maintain themolding material in a plasticized condition, (c) injecting saidplasticized molding material into a partially open mold cavity topartially fill the space between the mold faces, said mold cavity beingmaintained at a temperature sufficient to cure said molding material,(d) completely closing said mold faces to compress and cure the moldingmaterial in the internal configuration of said mold but not in saidtemperature controlled zone, and (e) remove the cured, molded articlefrom the mold.
 2. The method of claim 1 wherein the thermosettingmolding material is a phenolic resin molding composition.
 3. The methodof claim 1 wherein the thermosetting molding material is an epoxy resinmolding composition.
 4. The method of claim 1 wherein the thermosettingmolding material is an unsaturated polyester resin molding composition.5. The method of claim 1 wherein the thermosetting molding material is adiallylphthalate molding composition.
 6. The method of claim 1 whereinthe thermosetting molding material is a thermosetting natural orsynthetic rubber composition.
 7. The method of claim 1 wherein thethermosetting molding material is an amino resin molding composition.